1. Field of the Invention
The present invention relates to a process for conducting aldol condensation reactions and to the use of the aldol condensation products.
2. Description of the Background
Aldol condensations are important reactions which are conducted on an industrial scale. The xcex1,xcex2-unsaturated carbonyl compounds obtained from them are, because of their reactivity, starting materials for the synthesis of many organic compounds such as, for example, intermediates for the production of fragrances or pharmaceuticals. In addition, xcex1,xcex2-unsaturated aldehydes can be hydrogenated to form saturated aldehydes which can, inter alia, be oxidized to the corresponding carboxylic acids. These acids can be used for producing lubricants, desiccants, peresters or stabilizers for plastics. Complete hydrogenation of xcex1,xcex2-unsaturated aldehydes gives saturated primary alcohols which are employed for producing detergents and plasticizers or as solvents.
The aldol condensation is the reaction of two keto compounds (aldehyde or ketone) with elimination of water to form a compound which contains both an olefinic double bond and a carbonyl function. If, for example, only one aldehyde is used as starting material, an unsaturated aldehyde having twice the number of carbon atoms as the starting aldehyde is formed. This type of reaction is catalyzed by acids and bases. In industrial processes, preference is given to bases, especially inorganic bases such as NaOH.
By far the most important industrial process of this type is the condensation of n-butyraldehyde to form 2-ethylhex-2-enal, which is an intermediate for the preparation of 2-ethylhexanol, a plasticizer alcohol.
A possible way of conducting this reaction is described in the SRI-Report 21 C. The reaction is conducted in two condensation reactors connected in series. As catalyst, use is made of a two percent strength sodium hydroxide solution. The residence time in each of the two reactors is about 14 minutes. The reaction temperature is maintained at 85xc2x0 C. in the first reactor and 90xc2x0 C. in the second reactor by cooling, i.e. the reaction is not adiabatic and the heat of reaction has to be removed. The reaction mixture is subsequently separated into an aqueous catalyst solution and an organic phase by phase separation in a settling vessel. The catalyst phase is returned to the first reactor. Part of the catalyst is bled off to remove by-products and water of reaction and is replaced by fresh catalyst solution. The organic product phase which has been separated is washed free of base using water. The wash water is pumped to the first reactor. Water and n-butyraldehyde are separated from the crude product by distillation and are recirculated to the first reactor. The product which has been freed of these low boilers can be used as such or can be worked-up by distillation to give the pure product (2-ethylhex-2-enal).
As disclosed in DE 3530839, the condensation of n-butyraldehyde to form 2-ethylhex-2-enal is conducted in a flow tube at temperatures of 100-170xc2x0 C. under superatmospheric pressure in the presence of 0.5-5% strength sodium hydroxide solution as catalyst. The residence time is 0.2-5 minutes. After cooling to 60xc2x0 C., the reaction product is separated into the catalyst phase and product phase by phase separation. Part of the catalyst phase is bled off and replaced by fresh catalyst solution and the catalyst phase is then recirculated to the flow tube.
A disadvantage of the process is that water of reaction is removed by the discharge of catalyst solution. This stream is thus significantly larger than that which would be necessary purely for removing the carboxylic acids formed by the Cannizzaro reaction. This removal, therefore, results in a high catalyst consumption. The sodium hydroxide solution discharged contains organic compounds and, therefore, has to be worked-up or disposed of in an effluent treatment plant, thus incurring additional costs.
Decenal, a precursor for the plasticizer alcohol decanol (main constituent: isopropylheptanol), is prepared analogously to 2-ethylhexenal by aldol condensation of C5-aldehydes. Various processes for achieving this are described, for example, in DE 4 243 524, EP 562 450, EP 562 451, EP 646 563 and DE 4 243 524.
According to the disclosures of EP 562 451 and EP 646 563, the aldol condensation of valeraldehyde is conducted in a conventional manner, i.e. using a method analogous to the preparation of 2-ethylhex-2-enal described in SRI 21 C. It, therefore, suffers from the same disadvantages.
Another continuous aldol condensation process is disclosed in EP 634 994. This process comprises the following steps:
a) The starting aldehyde and the aqueous catalyst solution are fed into a stirred reactor operated under nonadiabatic conditions.
b) The reaction mixture obtained from the stirred reactor is introduced into the middle section of a distillation column.
c) The product obtained at the top of the distillation column is a gaseous mixture of starting material and water which, after condensation, separates into an upper organic phase and a lower aqueous phase.
d) Part of the aqueous phase is discharged.
e) The organic upper phase is recirculated to the reactor.
f) The bottom product obtained in the distillation is a mixture containing the aqueous catalyst solution, product and by-products (higher aldol addition or aldol condensation products, carboxylic acids and alcohols formed by the Cannizzaro reaction).
g) The bottom product is cooled.
h) The cooled bottom product separates into two phases. The upper organic phase contains the product, relatively high molecular weight products of further reactions and small amounts of catalyst solution. The lower phase comprises the aqueous catalyst solution which contains the carboxylic acid formed as by-product as salt and is saturated with product.
i) The catalyst phase which has been separated is recirculated to the reactor.
h) The product phase (upper phase) is taken off.
This process has a number of disadvantages:
a) The energy balance is capable of improvement, since the heat of reaction is not utilized. The heat of reaction has to be controlled by cooling the reactor, and the distillation of the reactor output requires energy. A cooling medium is needed to cool the bottom product obtained from the distillation.
b) During the distillation, the reaction mixture, which is basic because of the presence of the catalyst phase, is subject to thermal stress, which favors the formation of byproducts by the Cannizzaro reaction and thus reduces the yield. As a result, a larger amount of catalyst solution has to be bled off and replaced by fresh solution in order to keep the concentration of carboxylic acid salts constant.
c) The crude end product is taken from the plant without washing. It, therefore, still contains small amounts of catalyst, which constitutes a loss of catalyst. Furthermore, the entrained catalyst can cause deterioration of product quality during storage of the crude product. When using the product in a chemical synthesis, e.g. a hydrogenation, these catalyst residues can cause problems.
A need, therefore, continues to exist for an aldol condensation reaction which is industrially more acceptable for the product of condensation product.
Accordingly, one object of the invention is to provide a process for the condensation of keto compounds by the aldol condensation reaction to form xcex1,xcex2-unsaturated keto compounds which is more environmentally friendly and has better economics than the known processes.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for preparing xcex1,xcex2-unsaturated keto compounds by base-catalyzed aldol condensation of aldehydes and/or ketones having from 1 to 15 carbon atoms, comprising:
reacting the aldehydes and/or ketones with an aqueous catalyst solution under adiabatic reaction conditions; and
separating the reaction mixture obtained by rapid distillation into a top product comprising water, aldehyde and/or ketone and a bottom product comprising xcex1,xcex2-unsaturated keto compounds and aqueous catalyst phase.